Die quenching is well known, in which steel plate blanks are heated by energization heating and press-formed in a mold (for example, see JP 2008-87001 A). The blank to be press-formed is heated in advance so that the moldability is improved.
The blanks are heated above the predetermined temperature (where the austenaitc transformations occur), and the blanks are kept in contact with the cold mold, thereby quenching is performed with the press-forming.
In the respects of environment and safe, the products made of the steel plates for automotive applications have been high strength recently. However, the high strength plates need the guarantee in accuracy of connecting the multiple products. Moreover, in order to improve productivity and reduce the number of parts, the integration of multiple parts is required.
There are various techniques of answering such requirements, for instance, in order to integrate the multiple parts into one member, the high-strength blanks with desired shape (H-shape. T-shape or holed shape) are prepared, whereby the blanks with different shapes are heated and press-formed.
In order to heat the blanks with different shapes uniformly, heating the blanks for a long time in the heating furnace is not preferable because the facility and energy for the furnace would cost too much.
When the technique of JP 2008-87001 A is used to heat the blanks having the different shapes, in which the energization is operated from one end to the opposite end of the blank, there may be a variation in electric current flow at spaces between the electrodes where the section area changes largely. Thus, there may be a variation in current density in the blank, and it is difficult to obtain the even heating. To avoid such defectives, the multiple parts tire prepared for configuring the blank with the different shape, and the heating process and press-forming process is performed to each part, after that the multiple parts are combined into the blank.
Alternatively, JP 2002-248525 A discloses the technique of heating the blank with the different shape by energization heating, in which the multiple pairs of electrodes are connected to the opposite ends of the blank and used to energize the blank. Unfortunately, the technique of JP 2002-248525 A may fail to equalize the current density in the blank, because the current density largely changes at the portion where the section area perpendicular to the energization direction largely changes (e.g., if the blank has H-shape, the connection portions between the two parallel portions and the orthogonal portion).
As mentioned above, it is difficult to uniformly heat the blank that has the different shape in response to the recent requirement.